Reducing the effect of write disturbs in polymer memories

ABSTRACT

The write disturb that occurs in polymer memories may be reduced by writing back data after a read in a fashion which offsets any effect on the polarity of bits in bit lines associated with the addressed bit. For example, each time the data is written back, its polarity may be alternately changed. In another embodiment, the polarity may be randomly changed.

BACKGROUND

This invention relates generally to polymer memories.

In polymer memories, the polarization of a polymer may be altered bychanging the voltage applied across that polymer. An array of row andcolumn or bit lines may be arranged transversely to one another withpolymer material between those rows and columns at each row line/columnline intersection. The intersection of each row and column defines asingle memory element, or “pixel.” Any number of stacks of polymermemory layers may be combined to increase the memory capacity. Polymermemories are also called thin film electronics memory and polymerferroelectric random access memory.

Generally, the polarization of the memory pixel is achieved upon theapplication of an appropriate voltage. In the course of writing to agiven location, however, unaddressed bits on the same bit or column lineexperience a voltage that is less than the normal voltage used toachieve the desired polarization of a pixel. This voltage is called thewrite disturb voltage. The disturb voltage may be of either positive ornegative polarity.

For a small number of applied disturb voltage occurrences, due to asmall number of write sequences, an unaddressed pixel retains nearly allof its intended polarization. However, if a large number of writes occurthat affect the same unaddressed bit, and if those writes all have thesame polarity, the polarization of an unaddressed bit can be reduced tothe point where its content is corrupted. This may result in a biterror.

The problem is aggravated in polymer memories because polymer memoryread operations are destructive. As a result, after each read operation,the data is written back to the same location. As a side effect of theserepeated writes after each read, adjacent bits on the same bit lines inthe array may experience disturb voltages. The polarity of the disturbedvoltage is determined by the value of the bit being written on theassociated bit line. If an address is read many times, the data must bewritten back an equal number of times, all with the same polarity. Thecumulative effect of these write disturbs may degrade noise margin atother locations.

Thus, there is a need for a way to reduce the write disturb problem inpolymer memories.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of one embodiment of the presentinvention;

FIG. 2 is a schematic depiction of another embodiment of the presentinvention;

FIG. 3 is a schematic depiction of another embodiment of the presentinvention;

FIG. 4 is a flow chart for one embodiment of the present invention;

FIG. 5 is a schematic depiction of still another embodiment of thepresent invention;

FIG. 6 is a schematic depiction of still another embodiment of thepresent invention;

FIG. 7 is a flow chart for one embodiment of the present invention; and

FIG. 8 is a schematic depiction of one embodiment of the presentinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, a polymer memory 10 may include an exclusive or(XOR) gate 12 coupled to an inverter 14 that in turn couples to apolymer memory array 16, in one embodiment of the present invention. Inthe polymer array 16, an extra bit, referred to herein as a polaritybit, may be stored for each address in accordance with one embodiment ofthe present invention. The polarity bit may indicate whether the storeddata has been inverted. When the polarity bit associated with anyaddress is one, that may indicate that the data stored therein isinverted as one example. When that data is read, if the polarity bit isone, then the data may be inverted as part of the read process.

Thus, referring to FIG. 1, when the data is read, the polarity data maybe provided to one input of the exclusive or gate 12 and the actual datamay be provided to a different input of the exclusive or gate 12. Basedon the polarity data, a decision may be made whether to invert the readdata before outputting that data.

Conversely, if the polarity bit is zero, in one embodiment, the data isnot inverted and, again, the data arrives at the outputs in anon-inverted state. In one embodiment, although the scope of the presentinvention is not limited in this respect, when the data is written backafter a read operation, the data is inverted, as is the polarity bitthat is written back. Thus, as indicated in FIG. 1, when data is writtenback the data may be written through the inverter 14 to the data writeback port of the array 16, while a polarity indication is written backat the polarity write back port of the memory array 16.

This means that disturbs to other addresses, as a result of repeatedwrite backs, will be balanced when a location is repetitively read inone embodiment, although the scope of the present invention is notlimited in this respect. In other words, when the data is written backafter a read, its polarity may be reversed so that the effect of thewrite back on any bits in any associated bit lines is repeatedlyreversed, removing any cumulative disturb effect. The balanced disturbsthat result may not reduce noise margin in the way that unipolardisturbs do so, resulting in improved noise margin.

In some embodiments of the present invention the inversion may be doneon a random basis. Thus, instead of repeatedly alternating the polarity,the polarity may be changed in a random fashion. In this case, thepolarity may be randomly selected on each write back.

Referring to FIG. 2, in accordance with another embodiment of thepresent invention, a polarity bit may also be stored in the array 16 fora number of addresses. The polarity bit indicates whether the storeddata is inverted. A pseudorandom sequence generator 18 may be utilizedto control whether the inverter 14 inverts the write back data, or not.Because of the randomness of the signal from the generator 18, thepattern of inversion may be varied sufficiently to avoid unnecessarydisturbs when simple test patterns, like alternating ones and zeros areused. The probability of any sequence resulting in a stream of unipolardisturbs is the same as the probability of matching the pseudorandomsequence.

In accordance with another embodiment of the present invention, shown inFIG. 3, two global unipolar disturbs “D” may be applied to all pixels ofthe memory 10 b. The term “global” refers to a disturb applied to all orsubstantially all of the memory pixels. The term “disturb” refers to apositive or negative voltage applied to a pixel. One of the globalunipolar disturbs may be in each direction, the two global unipolardisturbs, in opposite directions, may be generated automatically every Nmemory accesses. This may reduce the likelihood that any pixel sees morethan N unipolar disturbs in the same direction.

For some polymer memories it may be observed that a string of greaterthan 64 unipolar disturbs may start to cause enough polarizationdegradation to raise concerns, but a disturb in the opposite directionmay restore the disturbed pixel to full charge. Thus, in one embodimentof the present invention, N is 64 and two unipolar disturbs are globallyapplied every 64 cycles, in the opposite directions, to break up anystring of unipolar disturbs in the same-direction, although the scope ofthe present invention is not limited in this respect. No actual read orwrite need be accomplished and the performance penalty may be relativelysmall in some embodiments.

Thus, as shown in FIG. 3, the control/signal generator 22 monitors forthe number of consecutive write backs. When that number has beenachieved, as determined by the generator 22, a global unipolar disturbof a first direction and a global unipolar disturb of the oppositedirection may be automatically generated as indicated at D.

Thus, in some embodiments, it may not be necessary to determine whethera string of N unipolar disturbs in the same direction has occurred. Theglobal disturbs may simply be accomplished regardless of the nature ofthe polarity of the write backs. In other embodiments, it may bedesirable to generate the opposite direction global disturbs when astring of a sufficient number of unipolar disturbs in the same directionhave been detected by the control/signal generator 22. Other variationsare also possible.

Referring to FIG. 4, in accordance with still another embodiment of thepresent invention, the memory array 16 may be periodically refreshed.Through the process of refreshing pixels in the entire memory array inseries at opportune times, a disturbed pixel may be restored to itsdesignated polarization state, either zero or one, although the scope ofthe present invention is not limited in this respect. In other words,the current state of a given pixel may be read and that state may bewritten back so that the proper voltage for that state is thenestablished at the pixel.

Referring to FIG. 4, the flow 30 begins by reading and writing regularlyin the array 16 as indicated at 32. When a read or write occurs, anaddress cycle counter may be incremented as indicated at block 34. Ifthe cycle counter is still less than a given number N, the flow iteratesat 36 through the blocks 32 and 34.

When the cycle counter equals N as indicated at 38, a given memorylocation is read and rewritten in a refresh operation as indicated inblock 40. In one embodiment, the first refresh may occur at a lowestaddressable address in the array, as one example. However, any techniquemay be used to select the initial address to refresh. Then, in oneembodument, a refresh location pointer may be incremented to point tothe next adjacent memory location as indicated in block 42. The addresscycle counter is reset to zero as indicated in block 44 and the flowiterates back to block 32. The next time the cycle counter equals N, theadjacent pixel is refreshed.

As a result, the entire array is periodically refreshed pixel by pixel,thereby progressively undoing any potential disturb effect, although thescope of the present invention is not limited in this respect.

Referring to FIG. 5, the memory array 16 of a polymer memory 10 c mayreceive data write back accesses and read data as indicated. A cyclecounter 24 may be positioned to count write backs and reads. A control26 may store information about a refresh location pointer 28. In oneembodiment, the pointer 28 information may be stored in the array 16itself. In addition, the control 26 may store code in a storage 30 tocontrol the refresh according to the flow shown in FIG. 4 in oneembodiment. In some embodiments, the storage 30 may also be part of thearray 16.

Referring to FIG. 6, a polymer memory 10 d may provide a compensatingdisturb prior to some or all memory cell writes in accordance with oneembodiment of the present invention. The cells of the memory 10 dincluded in the polymer memory array 16, including the actuallyaddressed memory cell, may be driven to experience a disturb voltage,with a polarity opposite to that experienced when the address memorycell write is performed. In one embodiment, the disturb voltage may bethe polarizing voltage V divided by three, although the scope of thepresent invention is not limited in this respect. As a result, adeterministically balanced sequence of positive and negative disturbsare experienced by all memory cells.

Thus, polymer memory array 16 may receive writes or compensatingdisturbs from the write state machine 46. The write state machine 46 mayreceive information about when write data is going to be written to thearray 16. The write state machine 46 may then control the writing so asto be preceeded by a compensating disturb.

Thus, referring to FIG. 7, the write state machine 46 may initiallydetermine whether there is a write to a particular address as indicatedin diamond 48. Pixels in the addressed column may then be precompensatedwith the appropriate disturb voltage as indicated in block 50.Thereafter, the addressed pixel may be written as indicated in block 52.

Referring to FIG. 8, a processor-based system 60 may include a processor62 coupled to an interface 64. The processor 62 may be a digital signalprocessor or a general purpose processor, to mention two examples. Theinterface 64 may be coupled to a bus 68. In one embodiment of presentinvention, the bus 68 may be coupled to a wireless interface 70. Thus,the system 60 may, in some embodiments, be a wireless interface forfacilitating wireless communications. However, non-wireless applicationsare also contemplated.

The system 60 may include the memory 10 which may be any of memoryillustrated in the preceding Figures, including the polymer memories 10,10 a, 10 b, 10 c, or 10 d.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: reading data from a polymer memory; writing thedata back to the memory after reading the data; and changing thepolarity of the data written back after reading the data.
 2. The methodof claim 1 wherein changing the polarity of the data written backincludes alternating the polarity when the data is written back.
 3. Themethod of claim 1 wherein changing the polarity of the data written backafter reading the data includes randomly changing the polarity of thedata written back after reading the data.
 4. The method of claim 1including providing a polarity bit to indicate the polarity of the datawritten back after a read.
 5. The method of claim 4 including repeatedlychanging the polarity of the polarity bit when the polarity of the datawritten back is changed.
 6. The method of claim 5 including storing thepolarity bit in the polymer memory.
 7. The method of claim 1 whereinchanging the polarity of the data written back after reading the dataincludes determining information about the number of times data has beenwritten back to the memory.
 8. The method of claim 7 including usingsaid information to determine whether to write back data automaticallyto said memory to correct any write disturb.
 9. The method of claim 8including automatically writing back data of a first polarity and asecond polarity after a predetermined number of write backs.
 10. Themethod of claim 9 wherein said predetermined number of write backs is apredetermined number of write backs of the same polarity.
 11. A polymermemory comprising: a memory array; and a device to change the polarityof data written in the array.
 12. The memory of claim 11 wherein saiddevice repeatedly changes the polarity when the data is written backafter reading the data in the array.
 13. The memory of claim 11 whereinthe device randomly changes the polarity of the data written back afterreading data in the array.
 14. The memory of claim 11 wherein saidmemory stores a polarity bit to indicate the polarity of data writtenback after a read.
 15. The memory of claim 14 wherein said devicechanges the polarity of the polarity bit when the polarity of the datawritten is changed.
 16. The memory of claim 15 wherein said devicestores the polarity bit in said array.
 17. The memory of claim 11wherein said device to automatically generate a unipolar disturb after apredetermined number of write backs.
 18. The memory of claim 11 whereinsaid device to generate two unipolar disturbs of opposite polarity aftera predetermined number of write backs.
 19. The memory of claim 11wherein said device to count the number of write backs.
 20. The memoryof claim 11 wherein said device generates a global disturb of a firstpolarity and a global disturb of the opposite polarity automaticallyafter a given number of write backs.